Incrementally variable bicycle transmission

ABSTRACT

An incrementally variable transmission, particularly suitable for a bicycle, having a variable-sprocket which comprises: A drive-flange and an indexing-flange mounted on an axle with a pair of sprocket-segment-planets and at least a pair of idler-planets sandwiched between the flanges, the planets adapted to expand and contract in response to a relative rotation of the flanges and thereby increase and decrease, respectively, the effective diameter of the variable-sprocket.

This application is a continuation in part of my pending applicationSer. No. 06/456/736 filed on Jan. 10, 1983, now U.S. Pat. No. 4,521,207which is herein being incorporated by reference, which is a continuationin part of my previous pending application Ser. No. 06/387,618 filed onJune 11, 1982, now U.S. Pat. No. 4,493,678, which is a continuation inpart of my earlier abandoned application Ser. No. 06/310,506 filed onOct. 13, 1981, now abandoned.

BACKGROUND AND OBJECTIVES OF THE INVENTION

Conventional ten speed bicycle transmissions consist of a chain wrappedaround a front double sprocket and a rear cluster containing fivesprockets, a front and rear derailleurs for derailing the chain from onesprocket to another in response to a rider moving two control leverswhich are connected to the derailleurs. The derailleurs are made of alarge number of delicate parts which need frequent maintenance andadjustment, and are easily damaged. The derailleur system is also hardto operate, it generates continuous noise and inefficiences in certaingears due to chain's misalignment. To reduce the misalignment thinsprockets and a narrow chain are used. This narrow chain is alsodesigned to withstand the repeated sharp double bend that is imposed bythe rear derraileur's tensioner, and it is manufactured without aconnecting link which would interfere with the adjacent sprockets andthe tensioner's cage. Because the popularity of the derraileur systemthis narrow chain is available throughout the industry and in everybicycle repair shop, and it is commercially advantageous to incorporatethis chain in any new competing transmission system.

Due to the derailleur system deficiencies several attempts have beenmade in the past to introduce different systems, some of which are basedon variable sprockets. An example is found in British Pat. No. 453712where two flanges are stacked together with the planets being sidesaddled in a cantilevered position. However, such an arrangement inducessevere stresses under working loads which lead to severe deformation ofthe variable sprocket, especially since the flanges have to beperforated and lose most of their structural integrity, and thesedeformations interfere with synchronization of thesprocket-segment-planet and the chain. Even with an inherently rigiddesign such as of the present invention, small misalignment of thevariable-sprocket relative to the rear sprocket occur due to productiontolerances which can be accentuated due to elastic deformation of thevarious components of the bicycle. This misalignment causes the chain tofeed onto the variable sprocket at an angle. When the planets are intheir expanded position they are spaced apart with the chain bridgingthe voids between them. When the chain is feeding onto the variablesprocket at an angle the chain tends to improperly engage with theplanets and ride on top of the sprocket-segment-planet's teeth sinceunlike a standard sprocket which provides continuous guidance to thefeeding chain, the voids allow the angular misalignment to be translatedto lateral displacement. The problem can be corrected by sandwiching theplanets between flanges which are sufficiently close to one another toalso guide the chain, however, when the planets are contracted and thechain has to reach deep between the flanges, close spacing of theflanges will cause the chain to rub against one of the flanges whenmisalignment of the variable-sprocket and the rear sprocket occurs.

The object of the present invention is to overcome these and otherproblems, and to provide a simple durable system which can beeconomically mass produced from a combination of metal and plasticmaterials.

SUMMARY OF THE INVENTION

An incrementally variable transmission, for providing a plurality ofdistinct transmission gear ratios, particularly suitable for bicycles.The transmission is based on a variable-sprocket comprising adrive-flange and an indexing-flange mounted on an axle with a pair ofsprocket-segment-planets and at least two idler-planets sandwichedbetween and slidably connected to the flanges forming a relatively rigidstructure. The planets are adapted to expand and contract in response toa relative rotation of the flanges and thereby increase or decrease theeffective diameter of the variable-sprocket (the effective diameter, asused herein, shall mean the length of chain pulled by the sprocket perrevolution, divided by 3.14).

In the present invention the location of the planets is such that whenthe sprocket-segment-planet engages with the chain it takes over thechain's load from the previously engaged sprocket-segment-planetallowing the previously engaged sprocket-segment-planet to becomedisengaged from the chain.

The present invention utilizes a narrow chain which is used inconjunction with derailleur systems. Using this narrow chain offersseveral technical and commercial advantages: The chain is universallyavailable throughout the industry, it is designed to withstand the sharpdouble bend that a tensioner imposes and it is manufactured without aconnecting link, features which are needed to make a chain work in aderailleur system and by coincidence, these features are also beneficialby the present invention.

The chain's narrowness dictates a correspondingly narrow teeth on thesprocket-segment-planet which calls for a more accurate alignment of thechain and the sprocket-segment-planet as they come into engagement, andthe present invention provides such alignment by means of guide platesformed on the top surface of the planets. The narrowness of the chainalso increases the stress in the sprocket-segment-planet's teeth, andsince in the present system the full load is often carried by a singletooth whose accurate shape is critical to assure positive engagementwith the chain, it is preferred to make the teeth from hard steel.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a side view of a bicycle equipped with an incrementallyvariable transmission according to the present invention,

FIG. 2 shows a portion of the bicycle and the variable-sprocket asviewed in the direction pointed by an arrow 2--2 marked on FIG. 1,

FIG. 3 shows a partially broken front view of the variable-sprocket

FIG. 4 shows a rear view of the variable-sprocket with three of the sixlocking means removed,

FIGS. 5, 6 and 7 show front, side and rear views, respectively, of asprocket-segment-planet,

FIGS. 8 and 9 show rear and side views, respectively, of a steel insertfitted with a cross pin, around which the sprocket-segment-planet ismolded,

FIGS. 10, 11 and 12 show front, side and rear views, respectively, of anidler-planet,

FIG. 13 shows a cross section of an area of the variable-sprocket whichcontains sprocket-segment-planet, as viewed along section line 13--13marked on FIG. 3, and,

FIG. 14 shows an enlarged area of the sprocket-segment-planet which ismarked on FIG. 5 by an eliptical phantom line.

DESCRIPTION OF THE SPECIFIC EMBODIMENT

The attached FIGURES show a bicycle (FIG. 1) having a frame 14, with aseat post member 55, crank-arms 12 and 12' which are integral with anaxle 13 (FIG. 2). The axle 13 is rotatably mounted in the frame by meansof ball-bearing 15 and 15' which are secured to the axle 13 bysnap-rings 27 and 27'. Rear and front wheels 16 and 16' are alsorotatably mounted to the frame 14 in a conventional manner. The bicycleis equipped with an incrementally variable transmission comprising avariable-sprocket 20 which is coupled by a rollerchain 32, preferably ofthe type used with conventional derailleur systems, to a sprocket 17which, in turn, is conventionally coupled to the rear wheel 16 via aone-way-clutch 18.

The sprocket 20 (FIGS. 2, 3, 4 and 13) has:

A drive-flange 21 molded onto the crank-arm 12 which has dimples 11 toimprove the mechanical bond and assure a rigid connection with theflange 21,

an indexing-flange 25, having a round bore 28, is rotatably mounted onthe axle 13 opposite the drive-flange 21 and has a plurality ofspiral-wavy-cams 26,

two sprocket-segment-planets 29 (identical elements, or identical partsthereof, will be identified by same numbers) and four idler-planets 31and 31' (planet 31' is a mirror image of planet 31) sandwiched betweenthe flanges 21 and 25,

keys 34 and 34' slidable in their respective radial keyways 24 and 24',for tangentially connecting the planets to the drive-flange 21,

cam-follower 37 attached to each of the planets 29, 31 and 31', forengaging with and following their respective spiral-wave-cam 26,

the spiral-wavy-cam comprising a series of alternating depressions 35and hills 36 (FIGS. 3 and 4). Whenever the cam-follower 37 moves fromone depression to the next, it moves over the hill 36 at which point itis further away from the axle 13 than when it is seated in either of thedepressions 35 between which it is moving. As the planet slides alongthe spiral-wavy-cam 26, from one depression to the next, the effectivediameter of sprocket 20 decreases and increases, respectively, inresponse to a relative rotation between the flanges 21 and 25. Lockingmeans 47 formed at the end of the camfollower 37 slides along andengages an outer surface of the indexing-flange 25. Under working loadswhich may cause a slight deformation of the indexing-flange 25, thelocking means 47 engages the outer surface of the indexing flange 25 andprevents the separation of the planets from it (the shape of the lockingmeans 47 allows their insertion through slots 26' for attaching them tothe indexing-flange 25). The sprocket-segment-planets 29 are equippedwith additional locking means, in the form of a hook 44 which slidesalong and engages an outer surface 45 of the drive-flange 21, to resisttogether with the key 34 the tendency of the sprocket-segment-planet 29to rotate (in a direction shown by circular arrow 61, FIG. 4) and twistdue to the tangential force that the chain's load creates on teeth 30.

The planets 29, 31 and 31' each have a small cantilever spring 22 (FIGS.3, 5 and 10) which is formed as a part of the key 34, to create apreload between the key 34 and the keyway 24 to prevent the planets fromdeveloping an annoying rattle while they are not engaged with the chain32.

The chain 32 (FIG. 4) is wrapped around approximately one half of thesprocket 20, and engages with the teeth 30 that are located on a roundedsection of the sprocket-segment-planets 29. Part of the leading teeth30' is removed to prevent interference with the chain 32 when theincrementally variable transmission is in high gear ratios. The chain 32is made of links and has a pitch length equalling the length of thechain 32 divided by the number of links contained in the chain. Itshould be noted that while a 10 speed bicycle's roller chain ispreferred, the term chain covers other chains and toothed belts whichcould also be used for putting together the system.

The flanges 21 and 25 are preferably molded from plastic material suchas, for example, polyester resin containing around 50% fiberglassreinforcement.

In the present invention the whole load is often carried by a singletooth 30, however, the width of the teeth 30 is limited by chain'snarrowness, therefore, it is prefered to form the teeth 30 on an insert63 made of a flat piece of hard steel (FIGS. 8 and 9). A cross pin 65 isinserted through the cam-follower, the insert 63 and the body of thesprocket-segment-planet, to strengthen it so that under load theaccurate geometry of the sprocket 20 as a whole will be maintained. Aswill be discussed later on, this contributes to the smooth operation ofthe sprocket 20. The sprocket-segment-planet 29 is preferablymanufactured by molding plastic material, for example, nylon with 43%fiberglass reinforcement, around the insert 63 and the cross pin 65.

The sprocket-segment-planet and the idler-planet each have a pair ofguide plates 62, 64 and 66, 67, respectively, on their top side todirect the chain onto the teeth 30. This is especially helpful in thecase of a narrow chain where any slight misalignment will cause thechain to ride on top of the teeth.

As the bicycle is peddled, load is developed in the chain 32 which inturn creates tangential and radial loads on the planets. The tangentialloads are taken by the sprocket-segment-planets 29 and are transmittedto the keys 34 and 34' which are formed on the side of each of thesprocket-segment-planets 29, and which engage with and slide in theradial keyway 24 and 24', respectively, transmitting these loads to thedrive-flange 21. Radial loads which develop in the planets around whichthe chain 32 is wrapped, are transmitted to the spiral-wavy-cams 26,through the cam-followers 37 formed on the sides of the planets 29, 31and 31'. The radial load secures and properly positions the planets inthe depressions 35.

Since only a small fraction of the periphery of the sprocket 20 carrieschain engaging teeth 30, it is preferred to design the shape of some ofthese teeth 30 so that they positively engage with rollers 32' of thechain, to prevent the chain from disengaging by escaping over the teeth,especially under dynamic loads which are associated with bicycling. Toachieve the positive engagement of the chain's rollers 32' the design ofthe standard tooth shape of a commercial roller chain sprocket ismodified (FIG. 14) by adding a section 50 which is designed between thephantom line 51 and line 52. The line 52 is the modified tooth shape. Aphantom line 51' and a line 52' mark imaginary inclined planes on whichthe roller 32' has to climb, in order to disengage from the tooth 30, inthe case of the commercial and the modified tooth shapes, respectively.The inclined plane 52' is so steep that it revents the load in the chainfrom pulling the roller 32' over the tooth 30, whereas experience hasshown that in the case of an unmodified commercial tooth shape theroller 32' may occasionally climb over the tooth.

The actual characteristics of the disengagement of thesprocket-segment-planet from the chain 32 may vary with the prevailingcoefficient of friction between the roller 32' and the tooth 30. It isnecessary to assure positive engagement with a low coefficient offriction, and to assure disengagement even when a high coefficient offriction prevails. Therefore, it is important to remove the load fromthe sprocket-segment-planet 29 which is about to disengage from thechain 32. The engagement of the one sprocket-segment-planet 29 (whichtakes place at the top of the sprocket 20) occurs slightly prior to thedisengagement of the other sprocket-segment-planet 29 (which takes placeat the bottom of the sprocket 20). Modifying thesprocket-segment-planets' geometrical locations by slightly displacingthe depressions 35 towards the axle 13 (relative to their theoreticalposition which corresponds to zero slack in the chain section which isengaged with and trapped between both sprocket-segment-planets 29)prevents the trapped chain from becoming taut. This assures that thechain's load has been transferred to the newly engagedsprocket-segment-planet 29 from the previously engagedsprocket-segment-planet 29, which becomes free to disengage from thechain 32. The small radial displacement of the depressions 35 which isrequired to assure the load transfer does not create excessive slack inthe trapped chain or interfere with the operation of the sprocket 20 andthe length of the trapped chain remains substantially an integral(natural) number of pitches when the sprocket 20 is operational in anyof the gear ratios. Therefore, the synchronization (that is, the propermeshing) between the chain 32 and the teeth 30 is maintained. Excessiveor insufficient slack would prevent proper synchronization of the chain32 with the sprocket-segment-planet 29 at the point of their engagement,causing the teeth 30 to hit the roller 32'. Further, insufficient slackwould not only prevent the engaging sprocket-segment-planet 29 fromassuming the chain's load but instead it would generate an additionalload in the trapped chain and between both of thesprocket-segment-planets 29, and may make the chain's disengagement fromthe sprocket 20 difficult. In view of the above, it can be appreciatedthat rigidizing the sprocket 20 in order to maintain the abovegeometrical relationships is essential and this is achieved by usingvarious measures such as reinforced plastic materials, using the insert63 and cross pin 65 as well as molding the drive flage directly onto thecrank-arm 12. All these measures cooperate to achieve a commerciallyacceptable system.

A brake assembly 38 (FIGS. 2 and 3) has arm 39 which is pivoted on astud 40 which is affixed to the frame member 55. The arm 39 has arounded pawl 46 which is adapted to engage and brake dimples 46' whichare formed on the periphery of the indexing flange 25 in response tobeing pushed by an outer jacket 41' of the cable 41 which is secured toa bracket 42 by means of crimped ferrule 54. The cable 41 and its jacket41' are connected, at their other end, to a conventional hand leverassembly 43 which the rider can depress in order to move the cable 41relative to the jacket 41', and thereby engage the brake 38. When theindexing flange 25 is braked and the drive-flange 21 is rotated throughthe crank-arms 12 and 12', forward and backward, intersections of thekeyways 24 and the spiral-wavy-cams 26 expand and contract together withthe planets, respectively, causing the transmission to change to a highgear ratio and a low ratio, respectively.

The spiral-wavy-cams 26 are arranged so that thesprocket-segment-planets 29 remain at substantially symmetricalpositions relative to the axle 13, so that an imaginary line drawnthrough the sprocket-segment-planets 29 will pass through the axle 13.Thereby, at least one sprocket-segment-planet 29 is engaged with thechain 32 at all times to maintain the power transmission between thesprockets 17 and 20, but most of the time only onesprocket-segment-planet 29 is engaged with the chain 32, allowing thetransmission to change to a higher gear ratio (if the thesprocket-segment-planets 29 were clustered together, periodically, nonewould be engaged with the chain 32 which wraps approximately one half ofthe sprocket 20). Thus, as the sprocket 20 rotates, the chain 32 isengaged with one sprocket-segment-planet 29 or with the other, with ashort transitional overlap during which both sprocket-segment-planets 29are engaged.

In order to make the load transition between thesprocket-segment-planets 29 smooth, it is preferred to make it while aminimum amount of power is transmitted through the chain 32. Thetransition occurs when the sprocket-segment-planets 29 are at theirtop/bottom position, and since a rider usually pedals lighter when thecrank-arms are at their top/bottom position, arranging the crank-arms 12and 12' and the sprocket-segment-planets 29 along the same imaginaryline will synchronize the minimum load condition and the loadtransition.

Each of the planets 29, 31 and 31', is engaged with its own respectivekeyways and the respective spiral-wavy-cam 26 which controls the radiallocation of the respective planet. Therefore, it is a designer's optionto maintain all the planets at equal distances from the axle 13 or tomove one opposing pair (planets 29 for example) further away from theaxle 13 so that an imaginary ellipse can be drawn through planets 29, 30and 31. When such a configuration of planets is orientated relative tothe crank-arms 12 and 12', in accordance with a certain bio-engineeringtheory, the utilization of rider's capabilities should be improved.Further, one of the planets 29 can be moved slightly further than theother in order to provide a slightly higher ratio when the rider'sstronger foot, usually his right foot, pedals down. The planets 29 willstill be maintained in substantially symmetrical positions relative tothe axle 13. However, such refinements are probably worthwhile only forpeople who ride a bicycle competitively.

A tensioner mechanism 23 takes up chain's slack which develops when theplanets are contracted.

The bicycle is ridden and pedaled like a conventional bicycle, with theimprovement that lowering the transmission ratio (which is the effectivediameter of the sprocket 20 divided by the effective diameter of therear sprocket 17) is done by back pedaling while energizing the brake38, thereby slowing the indexing-flange 25 relative to the drive-flange21 and causing the intersection of the keyways 24 and thespiral-wavy-cam 26 to radially move towards the axle 13, contracting theplanets 29, 31 and 31' and decreasing the effective diameter of thesprocket 20. The one-way-clutch 18 permits back pedaling even when thebicycle is stationary. Shifting to a higher ratio is achieved by brakingand slowing the indexing-flange 25 relative to the drive-flange 21 whilepedaling forward, expanding the planets and thereby increasing theeffective diameter of the sprocket 20. As the planets are expanded orcontracted, the cams 37 are moved from one depression 35 to the next andthe length of trapped chain increases or decreases, respectively, by anintegral (natural) number of pitches so that the length of the trappedchain remains substantially an integral number of pitches and,therefore, the sprocket-segment-planet 29 remains synchronized with thechain 32 at the point of engagement. When the planets shift over thehills 36, from one depression 35 to the next, the sprocket 20momentarily goes out of synchronization with the chain 32, but, as soonas the cams 37 are reseated in the depressions 35 the synchronization ofthe chain 32 with the sprocket-segment-planet 29 is reestablished,assuring their proper mesh. If the cams 37 are not properly seated, thechain load pushes them into depressions 35. It is easier to shift thesprocket 20, especially to a higher ratio, when the chain's load isminimal, that is when the only tension in the chain 32 is created by thechain tensioner 23. Shifting the transmission to a lower ratio can bedone while the bicycle is stationary, since the one-way-clutch 18 allowsback pedaling when the rear wheel 16 is not rotating, and this allows arider who stopped while being in a high ratio (at a traffic light, forexample) to shift to a lower ratio to ease acceleration. Shifting to ahigher ratio takes place while pedaling forward and when only onesprocket-segment-planet 29 is engaged with the chain, since during thetransition period when both sprocket-segment-planets 29 are engaged thetrapped chain momentarily prevents the planets from expanding.

While the present invention has been illustrated with one specificembodiment, it should be understood that modifications and substitutionscan be made without departing from the spirit of the invention or thescope of the claims.

I claim:
 1. A variable sprocket comprising in combination:an axle, adrive flange mounted on said axle, an indexing flange rotatably mountedon said axle opposite of said drive flange and having a plurality ofspiral-wavy-cams, two sprocket-segment-planets and at least twoidler-planets connected to said flanges, coupling means for transmittingforce between said sprocket-segment-planets and said drive flange, camfollowers attached to said planets for engaging with and following saidspiral-wavy-cams, said spiral-wavy-cams comprising a series ofalternating depressions and hills which move said planets closer andfurther to and from said axle, respectively, as said planets slide alongsaid spiral-wavy-cams, a chain made of links wrapping approximatelyone-half of the periphery of said variable-sprocket, saidsprocket-segment-planet having means for positively engaging said chain,said means comprising at least one tooth having an incline at a point ofcontacting said chain, said incline being steep so as to prevent saidchain from disengaging from said tooth while said tooth carries chain'sload, the improvement wherein the geometrical location of said planetsis slightly displaced toward the axle, relative to their theoreticalposition which corresponds to zero slack in said chain section which istrapped between said sprocket-segment-planets, so that when saidsprocket-segment-planet engages with the chain it takes over the chain'sload from the previously engaged sprocket-segment-planet allowing saidpreviously engaged sprocket-segment-planet to disengage from the chain.2. The subject matter of claim 1, wherein said sprocket-segment-planetsremain substantially symmetrical relative to said axle as they expandand contract so that at least one of said sprocket-segment-planets isengaged with said chain at all times and only one of saidsprocket-segment-planets is engaged with said chain part of the time. 3.The subject matter of claim 2, wherein the leading teeth are partiallyremoved to avoid improper engagement of these teeth with the chain whenthe planets are in an expanded position.
 4. The subject matter of claim1, wherein said sprocket-segment-planets are made of plastic materialand have a metalic insert on which chain engaging teeth are formed, atleast one of said teeth is shaped to positively engage said chain andremain engaged while chain's load is carried by said teeth.
 5. Thesubject matter of claim 4, wherein a metal cross pin is inserted throughsaid cam-follower to rigidize said sprocket-segment-planet.
 6. Avariable-sprocket as in claim 1, wherein at least one of said planetshas guide plates mounted on said top side, to guide said chain intoengagement with said planet.
 7. The subject matter of claim 1, whereinsaid drive flange is molded onto said crank-arm to rigidly connected oneto the other.
 8. The subject matter of claim 1, wherein saidsprocket-segment-planet and said crank-arms are arranged substantiallyalong the same line.
 9. The subject matter of claim 1,said drive flangehaving radial keyways a key attached to said sprocket-segment-planet forsliding in said keyway and transmitting tangential forces, an axleconnected to said drive-flange, an indexing-flange rotatably mounted onsaid axle opposite of said drive-flange and having spiral-wavy-cams, acam-follower attached to said planet for engaging with and radiallyfollowing said spiral-wavy-cam, said spiral-wavy-cam comprising a seriesof alternating depressions and hills which radially move the planetcloser and further to and from said axle, respectively, as said planetslides along said spiral-wavy-cam, the location of said planets beingdetermined by intersections of said keyways with said spiral-wavy-cams.10. The subject matter of claim 9, said planet having locking means toprevent it from separating from the spiral-wavy-cam under load.
 11. Thesubject matter of claim 9, said sprocket-segment-planet having lockingmeans to prevent it from rotating and twisting relative to the saiddrive-flange and also preventing the sprocket-segment-planet fromseparating from said drive-flange under load.